So the other day, I was moving a mini-fridge and didn't secure it. Sure enough, first corner I went around, it rolled over and broke one of the rear windows. The fixed glass was only on the 318is and M3, so it was much easier (and cheaper) to get the pop-out glass found on the 325. This howto describes the process for retrofitting the 318is with the pop-out style glass -- in this case, out of neccesity, although I think the pop-out style glass is probably an improvement anyway.
Parts required (sorry on no part numbers -- I got all this stuff from the Euro Depot junkyard and they included all the parts)
- Replacement rear window
- Weatherstripping for rear window
- Pop-out latch and mounting hardware (lever, mouting bar, mounting clips, and screws)
Start by removing the rear seat - pull up to remove the bench part from its clips:

Next, undo the two 10mm bolts on either side of the bottom of the back of the seat:

Now remove the side trim panel by gently prying it out with a screwdriver where the green clips are:

Unscrew and remove the coathook (phillips head screw) at the top, the seatbelt bolt (17mm hex bolt) in the middle, then remove the plastic cover that conceals the bottom window bolt at the bottom of the c-pillar.

Unscrew the two 8mm nuts that hold on the window -- one behind the coathook and the other behind the plastic cover at the bottom.

Remove the old window.

Put the new weatherstripping in, then bolt in the new window. Now for the tricky part -- cutting the fabric and exposing the area where the pop-out window bits mount. I started by peeling back the fabric -- there was a slight perforation in the yellow insulating foam around where the mount would be, but the mount wasn't there. I cut out the foam with a leatherman then used a screwdriver to scrape the adhesive off the frame:

Now insert the clips into the slots to the right of the holes as pictured:

The clips are threaded so you can run a screw through them. Put the bar in place and tighten the screws enough so you can move it around to get the positioning right, but not so much that it can't move.

Finally, maneuver the pop-out clip into position and assemble the pop-out mechanism into the window. Apologies on no picture of this, but it's actually rather straightforward.
Job complete! The procedure for replacing all the trim and seats is the opposite of removal.

One thing I didn't account for is that the "new" old window had oldschool chrome trim instead of black trim. Hopefully I'll be able to black it out so it doesn't look so goofy. Also, the weatherstripping at the top of the window wasn't included and was part of the single piece fixed window so I couldn't take it off. I may cut it off there or try to get some new weatherstripping that fits brand new elsewhere.